The present invention relates generally to firing devices of the type used to fire perforating guns used to perforate a cased well bore for the production of well bore fluids and, specifically, to a tubing pressurized firing device for firing a tubing conveyed perforating gun responsive to changes in the fluid pressure within the well bore tubing string.
As oil and gas well bores are drilled, the integrity of the borehole is preserved, usually by cementing a casing or liner in place in the borehole. The casing or liner is a metal, cylindrical conduit which must be punctured or perforated over the desired interval in order to produce well bore fluids once drilling is complete. A perforating gun which utilizes some form of fired projectile and an explosive charge is used to perforate the casing or liner to begin production from the well. Prior perforating gun techniques have either utilized tools which were run on a wireline or cable or were tubing conveyed devices which were run on a tubing string to the desired depth in the well bore.
Several problems exist with wireline run perforating gun systems. Wireline methods require a delicate balance between expected formation pressure and the drilling mud or fluid used to provide a balanced hydrostatic head in the well bore. A miscalculation of the expected formation pressure can result in a tangled wireline which necessitates pulling the tubing under adverse high pressure conditions. If an over balanced pressure condition exists during the perforation step, the well can loose large volumes of fluid into the surrounding formation which can damage the formation. Also, proper pressure differentials are needed to effectively clean the perforations. It is difficult with the wireline system to obtain the pressure differential needed to back surge the perforations and provide a clean well which will produce high yields. It is also difficult to use wireline systems in deviated well bores.
The tubing conveyed perforating gun generally allows immediate safe release of formation pressure at maximum pressure differentials into the tubing string. The back-surge which results tends to clean the perforation of mud filtrate, cement, and other perforating debris. With tubing conveyed perforating systems, the tubing can be run into position, a packer set to seal off a well bore, and the surface wellhead equipment can be installed. The packer setting can be checked by circulating fluid under pressure through the well annulus or through the well tubing string. Once the topside work is completed and tested for safety, the perforating gun can be fired to bring in the well. Since all surface work is completed before the perforating gun is fired, operating safety is enhanced.
In spite of these advantages, prior tubing conveyed perforating guns have lacked the ideal detonating system. Prior systems were usually detonated by (1) a bar dropped through the tubing string to fire a percussion detonator, (2) a through-the-tubing wireline connection system using an electrical charge to detonate the gun; or (3) well annulus pressurized systems which were actuated by fluid pressure acting through ports in the tubing string located above the packer. The first of these two systems required a wireline run in order to set a hydraulic packer to seal off the well bore. The third system mentioned above could only be used with a mechanically set well packer or a previously set permanent packer. Also, there was no satisfactory delay mechanism for the detonator of the gun.